There has hitherto existed a printed board in which a circuit composed of a copper foil is formed on a substrate by photolithography. Photolithography requires the step of etching a copper foil and high costs are required for a treatment of waste fluid generated by etching.
There has been known, as the technology requiring no etching, a method in which a conductive film is formed on a substrate using a copper particulate dispersion (copper ink) containing copper particulates (copper nanoparticles) dispersed in a dispersion vehicle (see, for example, Patent Document 1). According to this method, a liquid film of a copper particulate dispersion is formed on a substrate, and the liquid film is dried to form a copper particulate layer. The copper particulate layer undergoes photo sintering by irradiation with light, and thus a conductive film having low electric resistance is formed.
However, in the above-mentioned method, photo sintering may not sufficiently proceed even if energy of light irradiated in photo sintering is large, and thus failing to form a conductive film having low electric resistance.